Image transfer machines are used in printers, copy machines, facsimile machines, multi-function machines and the like. These machines utilize electrostatographic techniques to transfer an image from a toner-bearing photoreceptor surface to a transfer sheet passing over that surface. This transfer is most commonly achieved by electrostatic forces created by D.C. applied to or adjacent the back face of the transfer sheet while the front side of the sheet faces or contacts the photoreceptor surface. The transfer field is sufficient to overcome the forces holding the toner on the photoreceptor surface and to attract the toner onto the front face of the transfer sheet. These transfer fields are typically generated in one of two ways: by corona emission from a transfer corona generator; or by an electrically biased transfer roller or belt rolling along the back of the transfer sheet and holding it against the photoreceptor. The present disclosure relates to the electrical control of such transfer systems.
It is known that several factors contribute to affect the quality of the image transferred from the photoreceptor to the transfer sheet. Some of the factors are related to the components of the image transfer machine, such as the amount of useful life remaining in the component that generates the transfer field. Other factors are related to the environment in which the machine is being operated, namely, altitude, relative humidity and internal machine temperatures.
Still other factors are a function of the transfer sheet itself, such as paper weight or resistivity. Certain defects, known as white spots, arise when the toner particles are inadequately transferred from the photoreceptor surface to the face of the transfer sheet. White spot defects are especially sensitive to many of the above factors, especially where a high resistivity paper is being used.